Becoming a Follower of the Apocalypse has its perks: sanctuary, monetary compensation, and a new outfit. These are gained by supporting them in their humanitarian efforts.
The quest is given by Julie Farkas and is tied to High Times and Long-Term Care. In order to complete Friend of the Followers and be offered membership in the Followers, the player has to solve both of the related quests for the Followers (see the respective articles for more information) and donate enough medical supplies.
The player can donate Med-X, RadAway or Fixer. Once fifteen (15) items are donated (the game tracks all donations together, rather than separately for each type of medicine), Farkas will remark that the stock has greatly improved and the Followers now have a proper supply. She will discount medical services for the player and offer skill magazines for purchase.
In order for the Garrets to stock Absinthe and Rum & Nuka after completion of Long-Term Care, the player has to ask James Garret "How's business?" afterwards. Once he replies that the aforementioned drinks are in stock, they should be available for sale.
Although Julie Farkas mentions the Crimson CaravanCompany as a possible supplier, there are no dialogue options with anyone there.